This invention relates to a toner for developing static latent image to form a color image and to an image forming method using the toner.
Recently, an image forming method using a static latent image developing method such as an electrophotographic method is mainly used in an image forming apparatus for a high-speed and high-quality image formation.
The image formation process using the static latent image developing method is suitably applicable not only for forming a colored image but for stably attaining an image formation with a high-speed and high-quality for a long period.
However, some problems are raised when the developing method is applied to the color image formation, which are not solved yet even though various investigations have been performed.
Among the problems, the largest is as follows. Carbon black is usually used as a colorant in a black toner. The black toner tends to form fog under a condition with a high temperature and a high humidity since the resistively of the carbon black is lower than that of a colorant for a chromatic toner. Moreover, lowering of the static charged amount of the black toner during standing is large. Consequently, problems of character clogging, excessive increasing of halftone image and varying of gradation caused by excessive development density tend to be occurred when the toner is used for development after a long period of standing.
Besides, in the case of the chromatic toner, the toner amount for developing is reduced under a low temperature and low humidity condition compared with the black toner since the electro-conductivity of the chromatic toner is higher than that of the black toner. As a result of that, the balance of the black image and the chromatic image is lost so to lower the image quality.
In the case of the chromatic toner produced by a crushing method, the distribution of static charge amount is made broad since the distributions of the shape and particle size of such the toner are broad. Consequently, reproducibility of a fine dot and a fine line tends to be made insufficient since the scatter of the toner around the latent image.
Relating to such the problems, some countermeasures have been disclosed which have been based in the notice on the re-dispersion electro-conductivity, called also as a dispersion electro-conductivity. For example, Japanese Patent Publication Open to Public Inspection, hereinafter referred to as JP O.P.I., No. 7-319205 describes a toner having a re-dispersion electro-conductivity of within the range of from 1 to 100 xcexcS/cm. JP O.P.I. No. 2000-172007 describes a toner having a re-dispersion electro-conductivity of not more than 20 xcexcS/cm and a specified particle size distribution.
However, the above-mentioned problems have not been solved by such techniques and they still remain a problem in the art.
The object of the invention is to provide a toner for developing a static latent image to form a color image and an image forming method by which an image constantly showing no fogging, a high reproducibility of fine dot and fine line and a good color balance can be obtained.
It has been found by the inventors that there is a limit on the effect of the control of the re-dispersion electro-conductivity of the toner and a suitable full color image can be obtained by making a good balance between the static charge property of the black toner and the chromatic toner by controlling the re-dispersion electro-conductivity of the toners, the amount of free colorant on the black toner particle surface and the surface condition of the black toner.
1. A toner for developing static latent image to form a color image by combining chromatic toners of yellow, magenta and cyan and a black toner, wherein each of the toners is a toner produced by polymerization of a polymerizable monomer in an aqueous medium, the difference of the re-dispersion electro-conductivity of each of the toners is within the range of from 0.8 to 12 xcexcS/cm, the number of free colorant particles on the black toner surface is less than 9 per 500 toner particles, and a light absorbance at 500 nm of a black toner dispersion is not more than 0.08.
2. The toner for developing a static latent image, wherein xcfx81y greater than xcfx81bk, xcfx81m greater than xcfx81bk and xcfx81c greater than xcfx81bk, when the xcfx81y is the re-dispersion electro-conductivity of the yellow toner, the xcfx81m is the re-dispersion electro-conductivity of the magenta toner, the xcfx81c is the re-dispersion electro-conductivity of the cyan toner and the xcfx81bk is the re-dispersion electro-conductivity of the black toner.
3. The toner for developing a static latent image, wherein the each of the toners is a toner produced by the process for polymerizing the polymerizable monomer in the aqueous medium, salting/coagulating and washing.
4. The toner for developing a static latent image, wherein each of the chromatic toners has an average diameter of is from 3 to 8 xcexcm and a ratio of toner particles having a shape coefficient of from 1.2 to 1.6 of not less than 65%.
5. The toner for developing a static latent image, wherein each of the chromatic toners has the average diameter of from 3 to 8 xcexcm and a ratio of particles having no corner of not less than 50%.
6. The toner for developing a static latent image, wherein the sum M of a relative frequency m1 of toner particles included in the highest frequency class and a relative frequency m2 of toner particles included in the next frequency class is not less than 70% in a histogram showing the particle size distribution based on the number of the particles in which natural logarithm lnD of the particle diameter of each of the toners D xcexcm is taken on the horizontal axis and the axis is divided every 0.23.
7. An image forming method for forming a color image by a combination of yellow, magenta and cyan chromatic toners and a black toner, wherein each of the toners is a toner produced by polymerization of a polymerizable monomer in an aqueous medium, the difference of the re-dispersion electro-conductivity of each of the toner is within the range of from 0.8 to 12 xcexcS/cm, a number of free colorant particle on the black toner surface is less than 9 per 500 toner particles, and a light absorbance at 500 nm of a black toner dispersion is nor more than 0.08.
8. The image forming method for forming a color image, wherein xcfx81y greater than xcfx81bk, xcfx81m greater than xcfx81bk and xcfx81c greater than xcfx81bk, when the xcfx81y is the re-dispersion electro-conductivity of the yellow toner, the xcfx81m is the re-dispersion electro-conductivity of the magenta toner, the pa is the re-dispersion electro-conductivity of the cyan toner and the xcfx81bk is the re-dispersion electro-conductivity of the black toner.